Probing the environments surrounding ultrahigh energy cosmic ray accelerators and their implications for astrophysical neutrinos

TL;DR

This paper investigates the environments of ultrahigh energy cosmic ray sources and their connection to astrophysical neutrinos, finding photon-dominated sources fit observations better and emphasizing the importance of neutrino flux measurements.

Contribution

It demonstrates that photon-dominated source environments better explain UHECR data and explores the implications for neutrino observations and acceleration mechanisms.

Findings

Photon-dominated sources provide a better fit to UHECR data.

Gas-dominated sources are in tension with neutrino constraints.

UHECR sources can explain the high-energy neutrino spectrum above 1 PeV.

Abstract

We explore inferences on ultrahigh energy cosmic ray (UHECR) source environments -- constrained by the spectrum and composition of UHECRs and non-observation of extremely high energy neutrinos -- and their implications for the observed high energy astrophysical neutrino spectrum. We find acceleration mechanisms producing power-law CR spectra~$\propto E^{-2}$ are compatible with UHECR data, if CRs at high rigidities are in the quasi-ballistic diffusion regime as they escape their source environment. Both gas-dominated and photon-dominated source environments are able to account for UHECR observations, however photon-dominated sources give a better fit. Additionally, gas-dominated sources are in tension with current neutrino constraints. Accurate measurement of the neutrino flux at $\sim 10$ PeV will provide crucial information on the viability of gas-dominated sources, as well as whether diffusive shock acceleration is consistent with UHECR observations. We also show that UHECR sources are able to give a good fit to the high energy portion of the astrophysical neutrino spectrum, above $\sim$ PeV. This common origin of UHECRs and high energy astrophysical neutrinos is natural if air shower data is interpreted with the Sibyll2.3c hadronic interaction model, which gives the best-fit to UHECRs and astrophysical neutrinos in the same part of parameter space, but not for EPOS-LHC.